EVOLUTION
THE MULTITUDE OF LIVING BEINGS
PHYLOGENETIC TREE
• It shows the evolutionary relationships among
11 different vertebrates. This tree was
constructed by comparing the sequences of
the gene for cytochrome b, a protein involved in
energy metabolism.
• The 11 different animals have been positioned
in the tree according to the similarity of their
cytochrome b gene sequences.
• This tree is consistent with other information
(e.g., data obtained from the study of fossils),
except for the positions of the three fish
species.
SPECIATION
• It is an evolutionary process by which a new species comes into being.
• A species is a group of organisms that can reproduce with one another to produce fertile offspring
and is reproductively isolated from other organisms.
• Speciation can be driven by evolution, which is a process that results in the accumulation of many
small genetic changes called mutations in a population over a long period of time.
• There are a number of different mechanisms that may drive speciation. One of these is natural
selection, which is a process that increases the frequency of advantageous gene variants, called
alleles, in a population. Natural selection can result in organisms that are more likely to survive and
reproduce and may eventually lead to speciation.
• A second process called genetic drift describes random fluctuations in allele frequencies in
populations, which can eventually cause a population of organisms to be genetically distinct from
its original population and result in the formation of a new species.
GENETIC DRIFT
• Genetic drift takes place when the occurrence of variant forms of a gene, called alleles, increases and
decreases by chance over time. These variations in the presence of alleles are measured as changes
in allele frequencies.
• Typically, genetic drift occurs in small populations, where infrequently occurring alleles face a greater
chance of being lost. Once it begins, genetic drift will continue until the involved allele is either lost by a
population or until it is the only allele present in a population at a particular locus. Both possibilities
decrease the genetic diversity of a population. Genetic drift is common after population bottlenecks,
which are events that drastically decrease the size of a population. In these cases, genetic drift can result
in the loss of rare alleles and decrease the gene pool. Genetic drift can cause a new population to be
genetically distinct from its original population, which has led to the hypothesis that genetic drift plays a
role in the evolution of new specie